All modern cellular communication systems use control signaling for communicating common and user-specific information to the user equipments (UEs) in the system. When striving for a higher bit rate, modern cellular communication systems such as 3GPP LTE (Long Term Evolution) Rel-10 are now targeting so-called carrier aggregation. The aggregation can be performed at different layers in the protocol stack. LTE is typically targeting performing the aggregation above the hybrid-ARQ (hybrid Automatic Repeat reQuest) functionality, see 3GPP TS 36.212, “Multiplexing and channel coding”, V10.1.0 (2011-03). Moreover, for Rel-10 of the 3GPP specifications, each component carrier shall be accessible by Rel-8 terminals.
To support the transmission of downlink and uplink transport channels, there is a need for certain associated downlink control signaling. This control signaling is often referred to as the downlink L1/L2 (Layer 1/Layer 2) control signaling, indicating that the corresponding information partly originates from the physical layer (Layer 1) and the Layer 2 MAC. Typically, the L1/l2 control signaling is transmitted within the first part of each sub-frame. Thus, each sub-frame can be said to be divided into a control region followed by a data region, where the control region corresponds to the part of the sub-frame in which the L1/L2 control signaling is transmitted. For current LTE systems, the control signaling performed for each downlink sub-frame is located in the first n OFDM (Orthogonal Frequency Multiplexing) symbols, where n≦3. The downlink L1/L2 control signaling corresponds to three different physical channel types, namely the Physical Control Format Indicator Channel (PCFICH) informing the terminal about the size of the control region (one, two or three OFDM symbols), the Physical Downlink Control Channel (PDCCH) for signaling downlink scheduling assignments and uplink scheduling grants, and the Physical Hybrid-ARQ Indicator CHannel (PHICH) for signaling hybrid-ARQ acknowledgments in response to uplink UL-SCH transmissions. The PCFICH consists of two bits of information that are always mapped to the first OFDM symbol of each subframe in order to enable the user equipment to find the control channels, and to locate the start of the data region for the corresponding subframe.
In prior art, as represented by [1] two approaches for the downlink control signaling are described, namely:                1) A separate PDCCH (Physical Downlink Common Control Channel) per scheduled component carrier.        2) A single PDCCH containing information for all scheduled component carriers.        
For both these approaches, it is possible to transmit the single or the separate PDCCH on one of the component carriers only. In such a case, the carrier that carries the PDCCH (or PDCCHs) is referred to as the anchor carrier and the other (scheduled) carriers as extension carriers.
For current LTE systems, as mentioned previously, the control signaling performed for each downlink sub-frame is located in the first n OFDM symbols, where n≦3. The entire sub-frame typically comprises 14 OFDM symbols. The downlink control signaling consists of a format indicator to indicate the number of OFDM symbols used for control in the present sub-frame; scheduling control information (downlink assignments and uplink scheduling grant); and downlink ACK/NACK associated with uplink data transmission.
An example of how the number of simultaneous scheduling grants varies with the frequency of the carrier and with the size of the control region is shown below in Table 1. In the example #CCE/PDCCH=8 which is the maximal value, and the network node is configured with two antenna ports.
TABLE 1Control RegionControl RegionControl RegionSize 1Size 2Size 3 5 MHz01210 MHz13520 MHz2610
The above-described current PDCCH design can in some cases cause the system to become control channel limited, e.g., there is a need for more control signaling than there are available control resources. This may in turn result in a non-optimal use of the data region of each subframe. In other words, the lack of control resources will reduce the number of users scheduled in the sub frame, thus leaving part of the resources available for data transmission unused. Some specific examples of control channel limitations will be described below.
One example concerns the case when there is a large number of low data-rate UEs in the cell. In this scenario, there is a need to schedule a plurality of UEs at the same time, each with a small resource allocation. This typically happens when there are many VoIP (Voice over Internet Protocol) users or when there is a high level of machine-to-machine communication.
A further example concerns the case of cross-carrier scheduling, i.e. a UE reads the PDCCH on one carrier and is assigned physical resources on another carrier. In this case, the PDCCH load on the anchor carrier increases and there is a risk of PDCCH limitation.
Yet another example concerns the case where the anchor carrier has a narrow bandwidth e.g. is bandwidth limited. If one wants to introduce new features in the future then there is a possibility to introduce new non-backwards compatible extension carriers. In order to still support legacy terminals there is a need to have a legacy carrier. In addition, when the number of the legacy UEs decreases with time it is natural to reduce the bandwidth that is allocated to legacy UE support. Another reason for having a narrow band anchor carrier is for energy saving reasons. A narrow bandwidth carrier requires less transmission power than a wide band carrier does and hence it is possible to save energy when the traffic is low by turning off extension carriers and only keeping a narrow band carrier activated in order to ensure coverage.
If the system, due to the above mentioned problems, runs out of PDCCH resources whilst there are still physical downlink resources left on the PDSCH or physical uplink resources on the PUSCH that could have been assigned to active users, then that is a problem since those resources are wasted.
A previously presented solution to the above-mentioned problem is to extend the control region size, and has been discussed in the 3GPP. The solution uses a “secondary” PCFICH (transmitted as a regular Rel-8 PDCCH) that determines the size of the total PDCCH. This solution was regarded as too complicated since the UE needs to dynamically determined extension grants in two steps: first determine the total size of the PDCCH and then decode the “extension PDCCH”.
Due to the above-mentioned problems, there is a need for a method of extending the control region size without increasing the complexity of the UE processing, and at the same time enabling both legacy user equipments and new user equipments to properly interpret the control signaling in order to provide a more efficient utilization of the control and data resources.